The present Invention relates in general to novel fiber optic interferometric physical sensors and, more particularly, to novel fiber optic interferometric sensors which utilize light energy inputs at more than one wavelength.
Guided wave interferometers using single-mode fibers have been proven to be useful tools in detecting various physical quantities such as acoustic waves, magnetic fields, and electric fields. In order to achieve high sensitivity, the arm lengths of the interferometers are made long (&gt;10 meters). The long arm lengths, however, cause large phase drifts in the output signals, mainly due to changes in ambient temperature and pressure.
The optical output (P.sub.1) from an interferometer is represented by P.sub.1 =C.sub.1 +C.sub.2 cos .phi.(t), where C.sub.1 and C.sub.2 are constants and .phi. is the phase difference between two waves propagating along the two interferometer arms, respectively. The differential phase .phi. is composed of two parts: one part due to signal (.phi..sub.S) and the other part due to noise (.phi..sub.N). When the ambient temperature changes, for example, .phi..sub.N drifts at a rate of about 18000 degrees per meter per degree (.degree.C.). Since the sensitivity of the interferometer is proportional to .vertline.dP.sub.1 /d.phi..vertline.=.vertline.C.sub.2 sin .phi.(t).vertline., the sensitivity fluctuates wildly with temperature change. This causes signal fading problems and results in system down-time due to lack of sensor sensitivity which occurs at the condition .vertline.dP.sub.1 /d.phi..vertline.=0 when .phi.=m.pi., where m is an integer.
This problem has been approached in various ways. One solution utilizes a feedback scheme in which a piezoelectric cylinder is wound with a plurality of turns of optical fiber. A feedback signal is applied to the cylinder to cause stresses in the optical fiber resulting in optical path length changes in the fiber proportional to the feedback signal. Such a system is described by D. A. Jackson et.al. "Elimination of Drift in a Single Mode Optical Fiber Interferometer Using a Piezoelectrically Stretched Coiled Fiber", Applied Optics, Vol. 19, No. 17, pp. 2926-2929. The disadvantage of this approach is that it requires bulky piezoelectric cylinders which add volume and weight to the device. Other prior art solutions have similar disadvantages.